Antibiotics are the most common drug class to treat bacterial infections and are a key part of the pharmaceutical industry. There are high annual expenses for antibiotics of more than US$10 billion in the United States. The global expenses are much greater than this estimate and most of these expenses are for outpatient drug prescriptions. There is significant prescription and over-prescription of antibiotics for many different kinds of illnesses. Sometimes, the cause of the illness may have a low chance to be bacterial but doctors want to be careful and still give antibiotics to patients. This high usage of antibiotics is a major public health problem because it creates drug-resistant bacteria. These are bacteria that cannot be killed by common antibiotics and they can cause serious infections that spread in the body. There is significant concern about bacterial resistance, especially multidrug-resistant bacterial strains that are called “super bacteria.” The biggest issue is that infections caused by these drug-resistant bacteria cannot be treated by common antibiotics.
The market potential for treating drug-resistant bacterial infections is very large. Antibiotic-resistant bacterial strains are estimated to affect 2 million patients annually in the European Union, for example. In the United States, the cost of antibiotic resistance is more than US$20 billion per year and patients need an additional one to two weeks of hospital care. Among different antibiotic-resistant bacteria, methicillin-resistant S. aureus (MRSA) is well-known and has been identified as the leading cause of skin and soft-tissue infections, which include acute bacterial skin and skin structure infections (ABSSSIs). MRSA is easily transmitted through skin-to-skin contact. It also shows resistance towards many drugs that have been resurrected or developed as Gram-positive antibiotics. It is estimated that more than 53 million people worldwide are MRSA carriers. The annual cost of MRSA infections in USA alone is more than US$14 billion. While MRSA skin infections are merely one example, the numbers from this case alone support that there are significant human and economic costs from infections caused by multidrug-resistant bacterial strains.
It is important to note that the market need is unmet. The development of new antibiotics is a low priority for many companies because the treatment time is short and it is difficult to identify new classes of antibiotics. Between the 1960s and 2011, only four new classes of antibiotics were developed and marketed. At present, only four large pharmaceutical companies have active research and development programs to create new antibiotics. For comparison, in the 1980's, there were 20 large companies trying to develop new antibiotics. The drop in antibiotic research has three main reasons. It is difficult to find new classes of antibiotics because it was already significantly researched. There are also many generic antibiotics available over the counter and companies worry that these drugs create high competition that could decrease possible profits from a new antibiotic. It is very expensive and takes a long time to develop a new drug so that drug should have patent protection and fill a need that is unmet by generic antibiotics. In addition, it has been difficult to get new antibiotics approved by regulatory agencies, including the United States Food and Drug Administration (FDA).
Recently, there is more attention to the issue because the number of super bacteria is growing. The FDA created a new product category called the Qualified Infectious Disease Product (QIDP). Drugs in this category have a special designation from the FDA, which shortens the approval review process and increases the time (an additional 5 years) for exclusive marketing. These benefits are important because it means that new antibiotics can reach patients more quickly and there is more economic motivation for pharmaceutical companies to develop new antibiotics. One particular focus of need is on treating acute bacterial skin and skin structure infections, including regular and antibiotic-resistant strains. The current management for these infections is incision and drainage, and antibiotics may also be given if they can treat the bacterial infection. Because standard antibiotics are resistant, there are several other antibiotics that are used in such cases. Table 1 describes the antibiotics, including their mechanism and long-term challenges.
TABLE 1Summary of Antibiotics Used to Treat MRSA Skin Infections.DrugStatusIssueVancomycinTop choice antibiotic toPoor drug properties require intravenoustreat MRSA patients inadministrationhospitalsTrimethoprim-Used by clinical doctorsNo research supports effectiveness; does notSulfamethoxazolein MRSA treatmentsimprove clinical treatment outcomes(with drainage)ClindamycinUsed by clinical doctorsNo research supports effectiveness; does notin MRSA treatmentsimprove clinical treatment outcomes(with drainage)MupirocinUsed as a topicalDrug-resistant bacteria quickly emerge soantibiotic for MRSAcannot be used for a long time.treatmentFusidic AcidUsed as a topicalDrug-resistant bacteria quickly emerge soantibiotic for MRSAcannot be used for a long time and intreatmentcocktails.FinafloxacinApproved in 2014 underApproved for ear infections, member ofQIDP designationfluoroquinolone class that have serious sideeffects and fast bacterial mutationDalvanceApproved in 2014 underIs a second-generation Vancomyocin, and(dalbavancin)QIDP designationstill requires intravenous administration.Sivextro (tedizolidApproved in 2014 underMember of oxazolidinone class, andphosphate)QIDP designationantibiotic resistance has been reported.OrbactivApproved in 2014 underIs a second-generation Vancomyocin, and(oritavancin)QIDP designationstill requires intravenous administration.
As can be seen from these statistics, there are two major problems. There are a limited number of topical antibiotics, and all classes of antibiotics have problems with resistant strains quickly emerging. One alternative is antimicrobial peptides that are membrane-active but these peptides have not found wide application because they have relatively expensive production costs (due to lengthy amino acid sequences) and there were two failed clinical trials in the 90's which led already risk-averse pharmaceutical companies to stick with small molecule antibiotics. However, with the impending rise of drug-resistant bacteria, there is growing recognition of the need for new classes of antibiotics, and several classes of antibacterial peptides are now in early- and late-stage clinical trials. Of note, no membrane-active antibacterial peptide is currently under clinical development despite the promise of membrane interference to serve as the basis for long-term sustained therapies with high barriers to the emergence of drug-resistant bacterial strains.
The treatment of viruses faces similar challenges with the emergence and re-emergence of viruses fast eclipsing the rates at which new antiviral drugs can be developed. This gap has motivated the development of broad-spectrum antiviral drugs and the lipid bilayer of enveloped viruses is a key antiviral. Small molecules with membrane active behaviors targeting the virus envelope have largely indiscriminate activity, whereas antiviral peptides are more selective and have strong potential to form the basis of new antiviral therapies. Importantly, targeting the virus envelope is therapeutically attractive because there is a very high barrier to the development of drug-resistant virus strains. This is because the envelope is not encoded in the virus genome, but rather derived from host cells. Two antiviral peptides that target the virus envelope have been reported and can function as topical microbicides. However, these peptides completely lack antibacterial activity.
In order to motivate the clinical development of an anti-infective peptide, it is important to invent peptides that have broad-spectrum activity against both viruses and bacteria. In one preferred embodiment, to treat bacterial skin infections, an antibiotic with a topical formulation would be easy-to-use for patients and doctors. However, topical antibiotics have serious problems because bacteria can easily mutate. Other types of antibiotics used to treat MRSA skin infections require intravenous administration or have questionable efficacy. The market needs a new antibiotic that has long-term potential to address these problems. An ideal antibiotic would have i) a topical formulation; ii) a high barrier to mutations developing, iii) low side effects; iv) high efficacy; and v) a broad spectrum. With other proper suitable therapeutic properties, an antibiotic of this kind could be suitable for treating a wide range of bacterial, viral, and fungal infections in humans and animals through intravenous, intraperitoneal, subcutaneous, topical, oral, nasal, and other administration routes.
Zika Virus
An example of the growing need for anti-infective peptides is the ongoing Zika virus (ZIKV) epidemic, which has been declared a global public health emergency (Gulland, B M J 352, i657 (2016), Lucey and Gostin, JAMA 315, 865 (2016)). While ZIKV was discovered more than half a century ago, until recently, it was classified as a neglected tropical disease with limited geographical scope and few cases of human infection (Faye et al., PLoS Negl Trop Dis 8, e2636 (2014)). In 2007, more than 70% of the population on Micronesia's Yap Island became infected and it was the first episode of human infection outside of Africa or Asia (Duffy et al., New England Journal of Medicine 360, 2536 (2009)). In 2016, the global spread of ZIKV has reached epidemic levels across at least four continents and there is growing concern about the pathogenic clinical symptoms caused by circulating strains, including neurological damage such as Guillain-Barre syndrome (Cao-Lormeau et al., The Lancet, (2016)) and links between ZIKV infection and the rise of microcephaly among neonates (Mlakar et al., New England Journal of Medicine 374, 951 (2016)). Moreover, there are a number of possible transmission routes including mosquito vectors and human bloodborne and sexual transmission (Musso et al., Emerging Infectious Diseases 21, 359 (2015)). Given the accelerating pace of viral spread worldwide, the lack of countermeasures to prevent or blunt ZIKV infection is a major challenge, and there are currently no approved vaccines or therapies (Malone et al., PLoS Negl Trop Dis 10, e0004530 (2016)).
ZIKV is a member of the Flaviviridae family and is a mosquito-borne flavivirus related to the Dengue, Yellow Fever, Japanese Encephalitis and West Nile viruses (Chan et al., Journal of Infection, (2016)). Like other flaviviruses, ZIKV is an enveloped, positive strand RNA virus which possesses an approximately 11,000 base genome (Mukhopadhyay et al., Nature Reviews Microbiology 3, 13 (2005)). The RNA genome is packaged together with capsid proteins, and is enclosed within an icosahedral shell consisting of envelope (E) glycoprotein, membrane (M) protein, and precursor membrane (prM) protein that are embedded in a lipid bilayer (Lindenbach et al., Flaviviridae, p 712-746. Fields Virology 1, (2013)). Flavivirus particles exist in three forms—immature (noninfectious), mature (infectious) or host membrane-bound states—and the maturation process involves a structural transformation from a spiky to smooth surface morphology. It has long been known that infectious ZIKV particles are spherical with around 40-55 nm diameter (Dick, Transactions of the Royal Society of Tropical Medicine and Hygiene 46, 521 (1952)).
Recently, using cryo-electron microscopy, Sirohi et al. showed that the structure of ZIKV particles is similar to those of other flaviviruses, while noting that the ZIKV E protein has a distinct, highly variable region near the fusion loop that may influence sensitivity to antibodies (Sirohi et al., Science 352, 467 (2016)). Furthermore, Kostyuchenko et al. reported structural similarities between ZIKV and other flaviviruses, and also identified that ZIKV particles exhibit particularly high thermal stability due to a compact structure (Kostyuchenko et al., Structure of the thermally stable Zika virus. Nature advance online publication, doi:10.1038/nature17994 (2016)). Given the high structural stability of ZIKV particles, it has been suggested that therapeutic drugs which destabilize ZIKV particles would be useful agents for reducing disease outcome or limiting viral spread (Kostyuchenko et al., Structure of the thermally stable Zika virus. Nature advance online publication, doi:10.1038/nature17994 (2016)). Indeed, Wang and Shi have also recommended the development of flavivirus entry inhibitors that directly interfere with virus particles (Wang, ACS Infectious Diseases 1, 428 (2015)).
Towards this goal, a flavivirus broadly neutralizing antibody was demonstrated to bind to the ZIKV E protein of immature virus particles and protect against ZIKV infection, although this protection may occur through a complement-dependent effector function (Dai et al., Cell Host & Microbe 19, 696 (2016)). It has also been reported that ZIKV in plasma samples could be inactivated by exposure to a combination of photosensitive amotosalen intercalating agent and ultraviolet A illumination (Aubry et al., Transfusion 56, 33 (2016)). However, the identification of an antiviral agent that destabilizes ZIKV particles in the context of preventing virus entry remains elusive, particularly one which is both therapeutically selective and broadly applicable. In this regard, disruption of the lipid membrane surrounding enveloped viruses is an emerging approach towards developing virus entry inhibitors, and there is strong motivation to identify membrane-active compounds that achieve this goal with high selectivity and potency (Vigant et al., Nature Reviews Microbiology 13, 426 (2015), Jackman et al., “Nanomedicine for Infectious Disease Applications: Innovation towards Broad-Spectrum Treatment of Viral Infections,” Small 12, No. 9 (2016): 1133-1139, (2015)).